Possible impacts of spring sea surface temperature anomalies over South Indian Ocean on summer rainfall in Guangdong-Guangxi region of China

摘要

Based on observational and reanalysis data for 1979-2015, the possible impacts of spring sea surface temperature anomalies (SSTA) over the South Indian Ocean on the inter-annual variations of summer rainfall in Guangdong and Guangxi Provinces (i.e., the Guangdong-Guangxi area, GG) were analysed in this study. The physical mechanism behind these impacts was explored. Two geographic regions over [65A degrees E-95A degrees E, 35A degrees S-25A degrees S] and [90A degrees E-110A degrees E, 20A degrees S-5A degrees S] were defined as the western pole region and the eastern pole region, respectively, for the GG summer precipitation (P-GG)-related South Indian Ocean dipole SSTA pattern (R-SIODP). The difference between springtime SST anomalies averaged over the western pole region and that averaged over the eastern pole region was defined as the R-SIODP index. The correlation between the spring R-SIODP index and GG summer precipitation can reach up to 0.52. In the spring of positive R-SIODP anomaly, southerly winds over the western pole of the R-SIODP weaken, whereas the southeast trade winds over the eastern pole strengthen. By means of the wind-evaporation-SST feedback mechanism, the enhanced southeast trade winds can weaken the evaporation over the western pole of the R-SIODP and enhance the evaporation over the eastern pole. This results in a sustained positive SSTA in the western pole of the R-SIODP and a sustained negative SSTA in the eastern pole, whereby the distribution of the SSTAs maintains until summer. The SST dipole abnormally enhances the cross-equatorial airflow near 105A degrees E, which intensifies the anomalous anti-cyclonic circulation over South China Sea at 850 hPa and simultaneously results in abnormal enhancement of water vapour transport to GG. Additionally, the SST dipole promotes abnormal divergence in the lower troposphere and abnormal convergence in the upper troposphere over the maritime continent (MC) region. Moreover, the low-level convergence in GG is enhanced, which results in abnormal enhancement of ascending motion in the GG that is conducive to positive summer rainfall anomaly in this region. In this study, the spring R-SIODP index, the SST to the east of Australia and to the east of southern Africa, and the North Atlantic oscillation (NAO) index were used to construct a statistical prediction model for the inter-annual variability of the GG summer rainfall anomaly. This model can well predict the accuracy of the inter-annual variation of GG summer rainfall.